The preparation of relatively pure Group IV, V and VI metal oxides is important commercially. Niobium oxide is exemplary of this need as it is an important intermediate for the production of pure niobium metal and high purity, i.e., vacuum grade, ferroniobium. It is, therefore, desirable to obtain substantially pure niobium oxide for the subsequent reduction reaction to niobium metal or by metallothermic reaction with iron to form vacuum grade ferroniobium.
A ready source of niobium or tantalum is available in the form of metallurgical grade ferrometal alloys of these metals. For example, ferroniobium typically containing predominately iron and niobium and preferably more than 50% by weight niobium and most preferably between about 63% to 67% by weight niobium with the balance including iron and minor amounts of silicon and smaller amounts of tantalum, phosphorous and titanium is an attractive source of niobium which currently has not been commercially exploited. The presently available processes for the recovery of niobium oxide (Nb.sub.2 O.sub.5) from this source includes the chlorination of FeNb to produce ferric chloride (FeCl.sub.3) and niobium pentachloride (NbCl.sub.5) followed by a high temperature separation of the vapor phases of FeCl.sub.3 from the (NbCl.sub.5) by passing the vapor phases of these mixed chlorides through a bed of sodium chloride (NaCl) where the FeCl.sub.3 forms a solid eutectic composition with the NaCl, thereby effectively removing it from the vapor stream. Niobium chloride is then recovered by subsequently cooling the salt vapor to condense the (NbCl.sub.5). Conventionally, NbCl.sub.5 is then hydrolyzed by its addition to water which can then be neutralized, and the hydrolysis product dried, before calcining in a heated kiln in an oxygen containing atmosphere to produce relatively pure Nb.sub.2 O.sub.5. The drying and calcining is both energy intensive and expensive.
The preparation of Nb.sub.2 O.sub.5 by the previously described chlorination route utilizes toxic chlorine gas reacted exothermically at elevated temperatures and pressures. These conditions can produce severe corrosion problems. Special equipment is necessary for handling the highly pressurized, corrosive liquid chlorine and it must be safely vaporized, metered and fed into the reactor. Likewise, the most suitable material for large scale reactor construction is graphite. This is a brittle material which can fracture and fail abruptly after a short time in use in this environment. Further, the chlorine is normally used in excess to ensure complete reaction with the FeNb and the excess must be neutralized creating an expensive, undesirable by-product.
In addition to the foregoing, the hydrolysis step involves contacting the condensed chloride product with a neutralizing agent such as ammonia, and then filtering the resulting hydrous oxide slurry or cake, optionally drying and then air firing it to the oxide in a kiln. Such slurries and cakes are gelatinous and therefore hard to handle. There is need for a process which eliminates the problems associated with the reactants, reaction conditions, intermediate products and by products.